Solid oxide fuel cells have considerable interest in recent years, because of their high efficiency and environmentally friendly nature. Such systems required oxygen-conducting electrolytes and now the most common electrolyte is yttria stabilized zirconia (YSZ). This compound exhibits high oxide ion conductivity at elevated temperatures (850-1000 • C). However, this high working temperature causes problems in terms of materials selection and lifetime. One solution is to develop new oxide ions conductors exhibiting high oxide ion conductivity at intermediary temperatures (700-800 • C). Recent work has identified Ln 10−x Si 6 O 26±z (Ln = rare earths) as a good fast oxide ion conductor.Undoped and doped Ln 10−x B 6 O 26±z (B = Si or Ge) oxides are currently prepared by solid-state methods. In that work, we propose a sol-gel process to synthesize powders of La 9.33 Si 6 O 26 type-silicated apatites. The main advantage is to decrease the crystallization temperature in comparison to the conventional methods, allowing the synthesis of reactive powders with nanometric particles size. These oxides are synthesized using silicon alkoxide and lanthanum nitride as precursors. In the litterature, no study refers to the synthesis of mixed oxides with silicon alcoxides. However, there are several studies on sol-gel synthesis of glasses with this precursor. In this study, several processing parameters have been investigated (the hydrolysis ratio, the concentration of metallic precursors in the sol and the role of organic compounds) in order to synthesize pure phases after the decomposition of the sols. Pure powders of La 9.33 Si 6 O 26 type-silicated apatites are obtained at 800 • C.These powders were used to prepare ceramics. Several processing parameters as morphology of powders (agglomeration, particle sizes) and, heating profiles have been studied on the densification. Dense ceramics (90-95%) have been prepared at temperatures around 1400 • C. The used of sol-gel powders allow the decrease of the sintering temperature of about 200 • C.
Rhombohedral LaMnO 3+δ powders, prepared by two different soft chemistry routes (co-precipitation and hydrothermal synthesis), are sintered at 1400 • C for 2 h in air. Measurements of internal friction Q −1 (T) and shear modulus G(T), at low frequencies from −180 to 700 • C under vacuum, evidence three structural transitions of nearly stoichiometric orthorhombic LaMnO 3+δ . The first one, at 250 or 290 • C, depending on the processing followed, is associated to either a Jahn-Teller structural transition or a phase transformation from orthorhombic to pseudo-cubic. The second one at 610 or 630 • C is related to a phase transformation from pseudo-cubic or orthorhombic to rhombohedral. Below the Neel temperature, around −170 • C, a relaxation peak could be associated, for samples prepared according to both processing routes, to the motion of Weiss domains.
Perovskite oxides of formula Lal-xSrMnO 3 have been obtained by the thermal decomposition of precursor powders. Two different kinds of precursors, carbonates and citrates have been prepared by low temperature, i.e., "chimie douce" technique. The careful control of the chemical and the hydrodynamic parameters during the synthesis process allows obtaining nice homogeneous and small size particles (80 nm for the ex-carbonates and 30 nm for ex-citrates). Pure perovskite phase is observed after a low temperature thermal treatment, from 550 'C. The structure of these oxides is either rhombohedral or cubic and depends on the strontium content, the temperature and the partial pressure of oxygen during the thermal treatment. The Mn-O distances and the Mn-O-Mn angles are directly related to the amount of Mn 4 + content. INTRODUCTIONIn recent years, the doped perovskite manganites such as Lal-,SrMnO 3 have attracted growing attention due to the colossal magnetoresistance (CMR) properties. Several studies have shown that the microstructure, such as grain size, plays a very significant role in the intrinsic properties [ 1,2]. The synthesis methods are very important to obtain polycrystalline materials with specific microstructure.Traditional way of processing is usually the mixing of oxides, hydroxides or carbonates, followed by high temperature (T > 1000 'C) processing. Consequently, the materials obtained with these methods are constituted of large particles with low surface area. "Soft chemistry" techniques have been developed in order to obtain -at lower temperature -the same materials as the one observed at high temperature with the ability to control the particle size, the surface area and the stoichiometry of the powders [3]. The advantages of using powders of reduced grain size to prepare ceramics or thick films is the increase of the reactivity and the possibility of lowering the sintering temperature. The homogeneity is a very important parameter for having reproducible properties. Our previous studies performed on spinel manganites have evidenced the influence of the powder quality on the final material properties [4,5].The electronic properties are related to the mixed valence state Mn3+/Mn4+ that leads to mobile charge carriers. Low temperature synthesis allows moreover a larger oxidation state [6]. It was also observed that high oxygen ionic conductivity correlates with a cubic or an orthorhombic structure [7,8]. The coefficient of non stoichiometry 8 depends on the temperature of thermal treatment and on the oxygen partial pressure [9]. The highest values of 8 have been obtained for low temperature synthesis technique [10]. Many recent studies indicate that the LaMnO 3 + 8 compounds exhibit different structural types as orthorhombic Pnma, rhombohedral R3c,
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